The broad goal of this proposal is to understand how transcription is regulated by oxygen levels, an important response for helping cells adapt to oxygen availability. In S. cerevisiae, the ergosterol biosynthetic genes ERG2 and ERG6 are repressed by the Mot3 protein during hypoxic (low oxygen) conditions. Three specific aims will address the mechanism of this response. First, the Mot3 binding sites required for hypoxic repression will be determined through in vivo footprinting and promoter mutation studies. Second, oxygen-dependent regulation of Mot3 will be characterized through determination of post-translational modifications and protein binding partners. Third, factors required for hypoxic repression will be identified by mutant isolation and analysis. The findings here will have several implications for human health. First, the response to oxygen availability is important in many diseases; for example, in several forms of heart disease, certain tissues must cope with low perfusion and hence low oxygen levels. Second, ergosterol biosynthesis is an effective target of many antifungal agents. Finally, proper transcriptional control is critical in maintaining normal tissue growth, and disruptions can lead to such diseases as cancer.